﻿62 
  

  

  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  

  

  VOL. 
  88 
  

  

  sexes 
  are 
  suflScientlv 
  different 
  to 
  create 
  a 
  bimodal 
  curve, 
  the 
  males 
  

   being 
  the 
  smaller. 
  The 
  bimodal 
  curve 
  is 
  particularly 
  accentuated 
  in 
  

   Philippine 
  colonies 
  of 
  Oncomelania 
  quadrasi, 
  but 
  almost 
  absent 
  in 
  

   populations 
  of 
  O. 
  hupensis 
  in 
  China. 
  It 
  is 
  known 
  that 
  in 
  certain 
  

   marine 
  genera, 
  such 
  as 
  Stromhus^ 
  one 
  species 
  cliaracteristically 
  shows 
  

   sexual 
  dimorphism, 
  while 
  other 
  species 
  in 
  the 
  same 
  genus 
  do 
  not. 
  

  

  No 
  studies 
  have 
  been 
  undertaken 
  to 
  determine 
  what 
  role 
  is 
  played 
  in 
  

   causing 
  this 
  population 
  variation 
  by 
  such 
  factors 
  as 
  proportion 
  of 
  

   sexes, 
  population 
  size, 
  availability 
  of 
  food, 
  and 
  distribution 
  of 
  multi- 
  

   ple 
  alleles. 
  In 
  the 
  Philippines, 
  where 
  0. 
  quadrasi 
  lives 
  in 
  slow-run- 
  

   ning 
  creeks 
  and 
  where 
  little 
  or 
  no 
  migration 
  of 
  individuals 
  takes 
  

   place, 
  0.5 
  to 
  1.0 
  mm. 
  difference 
  in 
  mean 
  length 
  can 
  be 
  obtained 
  from 
  a 
  

   number 
  of 
  discreet 
  colonies 
  of 
  the 
  same 
  creek 
  system. 
  

  

  NO. 
  

   ADULTS 
  

  

  40t 
  CD 
  c? 
  56 
  

   32- 
  

  

  24 
  

  

  16- 
  

  

  8 
  

  

  0^ 
  

  

  MCAN 
  6'f6 
  KI»A 
  

  

  To+a! 
  le^ 
  

  

  4.5 
  50 
  

  

  i:l*i. 
  

  

  75 
  8«0 
  MM 
  

  

  5-5 
  60 
  6-5 
  70 
  

   LENGTH 
  OF 
  SHELL 
  

  

  Figure 
  11. 
  — 
  Histogram 
  of 
  the 
  shell 
  length 
  of 
  a 
  colony 
  of 
  adult 
  Pomatiofsis 
  lapidaria 
  

   (Say) 
  from 
  Ann 
  Arbor, 
  Mich. 
  Clear 
  areas, 
  denoting 
  males, 
  are 
  added 
  and 
  not 
  superim- 
  

   posed 
  on 
  shaded 
  areas 
  denoting 
  females. 
  

  

  Habitat 
  and 
  habits. 
  — 
  This 
  fresh- 
  water 
  species 
  is 
  even 
  more 
  amphibi- 
  

   ous 
  in 
  its 
  habitat 
  preference 
  than 
  6^ 
  nc(?wi^Zanm, 
  although 
  in 
  many 
  

   respects 
  it 
  could 
  be 
  called 
  the 
  '''' 
  Oncomelania^'' 
  of 
  North 
  America. 
  Like 
  

   Oncomelania., 
  it 
  spends 
  most 
  of 
  its 
  time 
  out 
  of 
  water 
  in 
  damp 
  vegeta- 
  

   tion 
  well 
  protected 
  from 
  direct 
  sunlight. 
  When 
  submerged 
  in 
  cool, 
  

   running 
  water, 
  the 
  animal 
  apparently 
  receives 
  enough 
  oxygen 
  through 
  

   its 
  gills, 
  and 
  so 
  it 
  makes 
  no 
  attempt 
  to 
  leave 
  the 
  water. 
  The 
  eggs 
  and 
  

   egg-la3dng 
  habits 
  of 
  this 
  species 
  are 
  as 
  yet 
  unknown, 
  although 
  they 
  

   probably 
  lay 
  eggs 
  on 
  moist 
  surfaces 
  at 
  the 
  water's 
  edge 
  as 
  do 
  their 
  rela- 
  

   tives, 
  Oncomelania 
  (see 
  Abbott, 
  1946). 
  P. 
  lapidaria 
  is 
  slightly 
  nega- 
  

  

  